1. Field of the Invention
The present invention relates to a camera, a solid-state imaging device used therefor such as a charge coupled device (CCD) type solid-state imaging device, and a method thereof.
2. Description of the Related Art
To achieve a larger angle of view and transfer at a high-speed rate of a CCD solid-state imaging device, there are demanded a lowering a resistance of transfer electrode. It is because such the transfer electrode is generally composed as a distributed constant circuit of an RC, and a high resistance of the transfer electrode will result in non-sharpness (dulling) and delay of a transfer pulse applied by the transfer electrode to disturb a transfer of CCD charges. Then, transfer electrodes and wiring bus lines have been made to be low resistance.
A technique of attaining a low-resistance transfer electrode is that, for example, when the transfer electrode is composed of polysilicon, an impurity is introduced to polysilicon to make the resistance low. Alternately, the polysilicon is made to be a thick film to obtain a low-resistance sheet. In those cases, it is expected an improvement up to only several tens of percentage both in the thickness and the resistance.
As another method of attaining a low-resistance transfer electrode, there is also known a method of using material having a low resistance for the transfer electrode instead of polysilicon. As the material to be used, tungsten silicide (WSi) is well known. In the case used WSi, the resistance is expected to become lower by about one digit.
For the case where the resistance has to be lowered by more than one digit, there has been proposed a configuration of forming a transfer electrode of the CCD itself by polysilicon and using material having a lower resistance than that of above explained WSi, such as aluminum, as a shunt wiring (for example, refer to the following publications: Japanese Patent No. 3123068, Japanese Unexamined Patent Publication No. 7-283387, Japanese Unexamined Patent Publication No. 7-226496, Japanese Unexamined Patent Publication No. 8-236743, and Japanese Unexamined Patent Publication No. 2003-60819).
Actually, most of the techniques so far have applied a method of providing a shunt wiring along a vertical transfer CCD. Such the shunt wiring in the vertical direction suffers from the disadvantages that the transfer mode is limited and multi-phase driving used for interleaving transfer of pixels is hard to be realized.
Furthermore, a configuration of connecting transfer electrodes made by polysilicon over several pixels in the crossing direction becomes also necessary. Although a sufficient thickness of polysilicon has to be secured and the polysilicon itself has to have a low resistance, for example, for performing high-speed driving, work of making pixels finer has a trade-off relationship with work of making the polysilicon film thicker. It is because when the polysilicon film becomes thicker, a height of light shading mask to be formed thereon becomes high, so that eclipse of light (meaning that a light to be irradiated on pixels is blocked by a light shading mask) becomes large when pixels become finer.
A CCD solid-state imaging device with finer pixels has been developed besides realization of a larger angle of view thereof, and a size of one pixel has become 2 μm or so nowadays. Although there are various challenges in realizing finer pixels, to maintain and improve sensitive characteristics is the most significant one.
In this case, since an aperture area of a light receiving portion is reduced due to the miniaturization of pixels, it is required the configuration of an upper layer portion of pixels, such as an on-chip lens, has to be optimized to improve the focusing property. However, an incident light to the light receiving portion is blocked by the transfer electrode itself, so that there has been a proposal of reducing a thickness and projections of the transfer electrode.
As a proposal for reducing the projections, instead of the vertical CCD configuration of forming a transfer electrode by two or three layers of polysilicon, a single-layer transfer electrode configuration of forming the same by one polysilicone layer has been proposed (for example, refer to Japanese Unexamined Patent Publication No. 2003-60819).
However, the circumstances are that a CCD with a larger angle of view and a CCD with high-speed transfer, etc. are actually hard to be realized only by the well-known single-layer transfer electrode configuration, and an eclipse of an incident light cannot be sufficiently reduced.